1. Field of the Invention
This invention relates to a suspension insulator for use in insulator strings to be supported by arms of transmission line towers. More particularly, the invention relates to a suspension insulator with an improved joint between an insulating member of the insulator and a metal pin thereof, so as to improve the electro-mechanical characteristics of the insulators.
2. Description of the Prior Art
Referring to FIG. 5, the porcelain insulating member of a suspension insulator has a shed 1a and a head portion 1c, and a metal cap 3 is firmly secured to the top of the head portion 1c by cement 2. A pin hole 1d is formed in the head portion 1c with a closed top and a downward opening. The inside surface of the pin hole 1d has a cylindrical surface portion S1, a curved top surface S3, and a round corner surface S2 forming a smooth transition from the upper end of the cylindrical surface portion S1 to the periphery of the curved top surface S3. In the sectional half of FIG. 5, the round corner surface S2 represents about one quarter of a circle. In the case of a conventional insulator of FIG. 5, sands 5 are rigidly deposited onto the entire inside surface of the pin hole 1d, so that a metal pin 4 can be firmly joined within the hole 1d of the head portion 1c by cement 2a.
The deposition of the sands 5 over the entire inner surface of the pin hole 1d provides very strong bondage between the insulating member head portion 1c and the metal pin 4. However, such conventional suspension insulator has a shortcoming in that the sands 5, at the round corner surface S2 tend to produce unstable concentration of electric field thereat, resulting in a considerable deterioration of the reliability of electric strength of the insulator. If one greatly reduces that inner surface of the pin hole 1d on which the sands 5 are deposited, the instability of electric strength can be eliminated but mechanical stress tends to be concentrated at the upper end portion of the area with the sands 5, and such mechanical stress concentration can cause a considerable reduction of the mechanical strength.